docs: def MappingField(cls, child_key, default=NOTHING, required=True, repr=False,
                   key=None):
      """
      Create new mapping field on a model.
  
      :param cls: class (or name) of the model to be related in Sequence.
      :param child_key: key field on the child object to be used as the map key.
      :param default: any mapping type
      :param bool required: whether or not the object is invalid if not provided.
      :param bool repr: include this field should appear in object's repr.
      :param bool cmp: include this field in generated comparison.
      :param string key: override name of the value when converted to dict.
      """
      default = _init_fields.init_default(required, default, OrderedDict())
      converter = converters.to_mapping_field(cls, child_key)
      validator = _init_fields.init_validator(required, types.TypedMapping)
      return attrib(default=default, converter=converter, validator=validator,
                    repr=repr, metadata=dict(key=key))